Compressed air systems, such as those used to operate the brakes of a heavy duty vehicle, commonly include an air dryer to remove moisture and oil entrained in the compressed air in order to protect the system from damage or failure. Air dryers used in such compressed air braking systems require a segregated quantity of air maintained in a purge volume for later backflow through the desiccant that removes the moisture from the air to purge the desiccant of the moisture. Purge volumes may be either an integral part of the air dryer, or a remote canister which is connected to the air dryer through an air line. Remote purge volumes are used where space constraints make it difficult or impossible to install an air dryer with an integral purge volume. It is also necessary to replace the desiccant periodically.